1. Field of the Invention
The present invention relates to display devices, driving methods for display devices, and electronic apparatuses, and more particularly, to a flat-screen (flat panel) display device in which pixels are two-dimensionally arranged in a matrix, a driving method for the display device, and an electronic apparatus including the display device.
2. Description of the Related Art
In recent years, in the area of display devices that display images, flat-screen display devices in which pixels (pixel circuits) are arranged in a matrix have rapidly become popular. Display devices including, as light-emitting elements of individual pixels, so-called current-driven electrooptical elements whose light emission luminances vary in accordance with the values of currents flowing in the elements, are one type of flat-screen display devices. In addition, organic electroluminescence (EL) elements exhibiting a phenomenon of emitting light when an electric field is applied to an organic thin film, are one type of current-driven electrooptical elements.
Organic EL display devices including organic EL elements as electrooptical elements of individual pixels have the features described below. That is, since organic EL elements are capable of being driven at an applied voltage of 10 V or less, a decrease in power consumption can be realized by using organic EL elements. Organic EL elements are light-emitting elements. Thus, compared with liquid crystal display devices in which the intensity of light from a light source is controlled using liquid crystals for each pixel so that images can be displayed, organic EL elements achieve high image visibility. Furthermore, since illumination equipment, such as a backlight, is not necessary, reductions in weight and thickness can be easily achieved. Moreover, since the response speed of organic EL elements is very high, such as about several microseconds, no afterimage is produced in the case of displaying moving images.
As in liquid crystal display devices, organic EL display devices can employ, as driving methods for organic EL display devices, a simple (passive) matrix method and an active matrix method. However, although simple-matrix display devices have a simple configuration, a light-emission period of an electrooptical element is shortened as the number of scanning lines (that is, the number of pixels) increases. Thus, it is difficult to realize a large-sized and high-definition display device.
Under such circumstances, in recent years, the development of active-matrix display devices in which currents flowing in electrooptical elements are controlled by active elements, such as insulated-gate field-effect transistors, provided in the same pixels where the corresponding electrooptical elements are provided, has been extensively conducted. As insulated-gate field-effect transistors, in general, thin-film transistors (TFTs) are used. In active-matrix display devices, electrooptical elements maintain light emission during a one-frame period. Thus, a large-sized and high-definition display device can be easily realized.
In general, the current-voltage characteristics (I-V characteristics) of organic EL elements deteriorate as the time passes (deteriorate with time). In particular, in a pixel circuit including an N-channel TFT as a transistor that drives an organic EL element by using a current (hereinafter, referred to as a “driving transistor”), when the I-V characteristics of the organic EL element deteriorate with time, the gate-source voltage Vgs of the driving transistor also varies. As a result, the light emission luminance of the organic EL element varies. This is because the organic EL element is connected to the source electrode side of the driving transistor.
More specifically, the source potential of a driving transistor is determined in accordance with an operating point of the driving transistor and an organic EL element. When the I-V characteristics of the organic EL element deteriorate, the operating point of the driving transistor and the organic EL element varies. Thus, even if a constant voltage is applied to the gate electrode of the driving transistor, the source potential of the driving transistor varies. Hence, since the source-gate voltage Vgs of the driving transistor varies, the value of a current flowing in the driving transistor varies. As a result, since the value of a current flowing in the organic EL element also varies, the light emission luminance of the organic EL element varies.
In particular, in a pixel circuit including a polysilicon TFT as a driving transistor, in addition to a deterioration in the I-V characteristics of an organic EL element with time, the transistor characteristics of the driving transistor vary with time and the transistor characteristics differ for each pixel depending on the manufacturing process. That is, the transistor characteristics of a driving transistor are different for each pixel. The transistor characteristics include, for example, the threshold voltage Vth of the driving transistor, the mobility μ of a semiconductor thin film forming a channel of the driving transistor (hereinafter, simply referred to as “the mobility μ of a driving transistor”), and the like.
In a case where the transistor characteristics of a driving transistor are different for each pixel, the value of a current flowing in the driving transistor is different for each pixel. Thus, even if the same voltage is applied to the gate electrodes of the driving transistors in individual pixels, a pixel-to-pixel variation occurs in the light emission luminance of the organic EL element. As a result, the uniformity (evenness) of a screen is degraded.
Thus, in order to maintain a constant light emission luminance of an organic EL element without the light emission luminance being affected by a deterioration in the I-V characteristics of the organic EL element with time, a variation in the transistor characteristics of a driving transistor with time, and the like, a pixel circuit has various correction (compensation) functions (for example, see Japanese Unexamined Patent Application Publication No. 2006-133542).
The correction functions include, for example, a compensation function for a variation in the characteristics of an organic EL element, a correction function for a variation in the threshold voltage Vth of a driving transistor, a correction function for a variation in the mobility μ of a driving transistor, and the like. Hereinafter, the correction of a variation in the threshold voltage Vth of a driving transistor will be referred to as “threshold correction”, and the correction of a variation in the mobility μ of a driving transistor will be referred to as “mobility correction”.
As described above, since each pixel circuit has various correction functions, the light emission luminance of an organic EL element can be maintained constant without the light emission luminance being affected by a deterioration in the I-V characteristics of the organic EL element with time and a variation in the transistor characteristics of a driving transistor with time. As a result, the display quality of an organic EL display device can be improved.